Two distinct types of T lymphocytes are recognized: CD8+ cytotoxic T lymphocytes (CTLs) and CD4+ helper T lymphocytes (TH cells). CTLs recognize and kill cells which display foreign antigens of their surfaces. CTL precursors display T cell receptors that recognize processed peptides derived from foreign proteins, in conjunction with class I MHC molecules, on other cell surfaces. This recognition process triggers the activation, maturation and proliferation of the precursor CTLs, resulting in CTL clones capable of destroying the cells exhibiting the antigens recognized as foreign.
TH cells are involved in both humoral and cell-mediated forms of effector immune responses. With respect to the humoral, or antibody, immune response, antibodies are produced by B lymphocytes through interactions with TH cells. Specifically, extracellular antigens are endocytosed by antigen-presenting cells (APCs), processed, and presented preferentially in association with class II major histocompatibility complex (MHC) molecules to CD4+ class II MHC-restricted TH cells. These TH cells in turn activate B lymphocytes, resulting in antibody production.
The cell-mediated, or cellular, immune response, functions to neutralize microbes which inhabit intracellular locations. Foreign antigens, such as, for example, viral antigens, are synthesized within infected cells and presented on the surfaces of such cells in association with class I MHC molecules. This, then, leads to the stimulation of the CD8+ class I MHC-restricted CTLs.
Some agents, such as mycobacteria, which cause tuberculosis and leprosy, are engulfed by macrophages and processed in vacuoles containing proteolytic enzymes and other toxic substances. While these macrophage components are capable of killing and digesting most microbes, agents such as mycobacteria survive and multiply. The agents' antigens are processed, though, by the macrophages and presented preferentially in association with class II MHC molecules to CD4+ class II MHC-restricted TH cells, which become stimulated to secrete interferon-γ, which, in turn, activates macrophages. Such activation results in the cells' exhibiting increased bacteriocidal ability.
TH cells are composed of at least two distinct subpopulations, termed TH1 and TH2 cell subpopulations. Evidence suggests that TH1 and TH2 subtypes represent extremely polarized populations of TH cells. While such subpopulations were originally discovered in murine systems (reviewed in Mosmann, T. R. and Coffman, R. L., 1989, Ann. Rev. Immunol. 7:145), the existence of TH1- and TH2-like subpopulations has also been established in humans (Del Prete, A. F. et al., 1991, J. Clin. Invest. 88:346; Wiernenga, E. A. et al., 1990, J. Imm. 144:4651; Yamamura, M. et al., 1991, Science 254:277; Robinson, D. et al., 1993, J. Allergy Clin. Imm. 92:313). While TH1-like and TH2-like cells can represent the most extremely polarized TH cell subpopulations, other TH cell subpopulations, such as TH0 cells (Firestein, G. S. et al., 1989, J. Imm. 143:518), which represent TH cells which have characteristics of TH1 and TH2 cell subpopulations.
TH1-like and TH2-like cells appear to function as part of the different effector functions of the immune system (Mosmann, T. R. and Coffmann, R. L., 1989, Ann. Rev. Imm. 7:145). Specifically, TH1-like cells direct the development of cell-mediated immunity, triggering phagocyte-mediated host defenses, and are associated with delayed hypersensitivity. Accordingly, infections with intracellular microbes tend to induce TH1-type responses. TH2 cells drive humoral immune responses, which are associated with, for example, defenses against certain helminthic parasites, and are involved in antibody and allergic responses.
It has been noted that the ability of the different TH cell types to drive different immune effector responses is due to the exclusive combinations of cytokines which are expressed within a particular TH cell subpopulation. For example, TH1 cells are known to secrete interleukin-γ (IL-2), interferon-γ (IFN-γ), and lymphotoxin, while TH2 cells secrete interleukin-4 (IL-4), interleukin-5 (IL-5), and interleukin-10 (IL-10).
It is thought that TH1 and TH2 subpopulations arise from a common naive precursor (referred to as THP). For example, naive CD4+ cells from mice which express a single transgenic T cell receptor can be induced to develop into either the TH1 or TH2 cell type. The conditions of antigen stimulation, including the nature and amount of antigen involved, the type of antigen-presenting cells, and the type of hormone and cytokine molecules present seem to all represent determinants of the pattern of TH1 versus TH2 differentiation, with, perhaps, the decisive role belonging to the cytokines present. With such a complex series of possible determinants, a full accounting of the exact factors important in driving TH1 or TH2 differentiation are, as yet largely unknown.
Further, it has recently been noted that, in addition to CD4+ TH cells, CD8+ CTLs can, under certain conditions, also exhibit TH1-like or TH2-like cytokine profiles (Seder, R. A. et al., 1995, J. Exp. Med. 181:5–7; Manetti, R. et al., 1994, J. Exp. Med. 180:2407–2411; Maggi, E. et al., 1994, J. Exp. Med. 180:489–495). While the precise functional role of such CD8+ TH-like cells is currently unknown, these cell subpopulations appear to have great relevance to immune responses against infectious agents such as viruses and intracellular parasites.
Once TH1 and TH2 subpopulations are expanded, the cell types tend to negatively regulate-one another through the actions of cytokines unique to each. For example, TH1-produced IFN-γ negatively regulates TH2 cells, while TH2-produced IL-10 negatively regulates TH1 cells. Moreover, cytokines produced by TH1 and TH2 antagonize the effector functions of one another (Mosmann, T. R. and Moore, 1991, Immunol. Today 12:49).
Failure to control or resolve an infectious process often results from an inappropriate, rather than an insufficient immune response, and can underlie a variety of distinct immunological disorders. Such disorders can include, for example, atopic conditions (i.e., IgE-mediated allergic conditions) such as asthma, allergy, including allergic rhinitis, dermatitis, including psoriasis, pathogen susceptibilities, chronic inflammatory disease, organ-specific autoimmunity, graft rejection and graft versus host disease. For example, nonhealing forms of human and murine leishmaniasis result from strong but counterproductive TH2-like-dominated immune responses. Lepromatous leprosy also appears to feature a prevalent, but inappropriate, TH2-like response.
It is possible that another example can be HIV infection. Here, it has been suggested that a drop in the ratio of TH1-like cells to other TH cell subpopulations can play a critical role in the progression toward disease symptoms. Further, it has been noted that, at least in vitro, TH2-like clones appear to be more efficient supporters of HIV viral replication than TH1-like clones.
Further, while TH1-mediated inflammatory responses to many pathogenic microorganisms are beneficial, such responses to self antigens are usually deleterious. It has been suggested that the preferential activation of TH1-like responses is central to the pathogenesis of such human inflammatory autoimmune diseases as multiple sclerosis and insulin-dependent diabetes. For example, TH1-type cytokines predominate in the cerebrospinal fluid of patients with multiple sclerosis, pancreases of insulin-dependent diabetes patients, thyroid glands of Hashimoto's thyroiditis, and gut of Crohn's disease patients, suggesting that such patients mount a TH1-like, not a TH2-like, response to the antigen(s) involved in the etiopathogenesis of such disorders.
A primary goal, for both diagnostic and therapeutic reasons, therefore, would be the ability to identify, isolate and/or target members of a particular TH cell subpopulation. The ability to identify those genes which are differentially expressed within and/or among such TH cell subpopulations is required to achieve such a goal. To date, investigations have focused on the expression of a limited number of specific known cytokines and cytokine receptors in the TH cell population. Cytokines, however, exert effects on cell types in addition to specific TH cell subpopulations, i.e., exhibit a variety of pleiotropic effects. It would be beneficial, therefore, to identify reliable markers (e.g., gene sequences) of TH cell subpopulations whose effects are TH cell subpopulation specific, e.g., which, unlike secreted cytokines, are TH cell subpopulation specific.